The Contribution of Sir George Airy to Positional Astronomy

Talk given by Gilbert Satterthwaite to the July Meeting of the Society

Gilbert Satterthwaite had the distinction of being the last person at Greenwich to take an observation using The Airy Transit Circle Telescope in 1954 before the Observatory was eventually moved to Herstmonceux in East Sussex.  He had taken up his post at Greenwich straight from school in 1952 and used instruments and books created by George Airy and so gained a great deal of respect for the former Astronomer Royal; in fact Gilbert referred to himself as a bit of a nut about Airy.  Gilbert is also Chairman of the Society for History of Astronomy and a member of the International Astronomical Union.

He began by looking at the problems associated with defining the positions of the stars and planets in the heavens.  Since we observe from the surface of the earth and not at the centre, every observer sees the night sky slightly differently and star positions need to be precisely positioned in a way that is recognised by all.

Gilbert explained definition of the Celestial Equator, the projection of the earth's equator onto the background sky.  The ecliptic is the projection of the earth's orbit around the Sun and the point at which they cross with the ecliptic rising is called the First Point of Aries and defines zero hour on the sidereal clock.

Every observer has their own meridian, a line from due south, passing directly over their head at its zenith and on to due north.  The Prime Meridian is the meridian at Greenwich Observatory as decided in 1884.  The Alt-Azimuth system is used when considering the meridian and since the horizon is not always clear, altitude readings are made from the zenith at 90 this angle is known as the zenith distance.

The position of a star is measured as it transits due south, when its zenith distance is also measured and its precise azimuth determined. 

To look at the work done on Positional Astronomy, Gilbert introduced us first to Friedrich Bessel, a German mathematician-astronomer who worked on the observations made earlier by James Bradley, a former Astronomer Royal, to produce precise positions of over 3,000 stars.

Next we were told about Stephen Groombridge, a business man and astronomer, born in Goudhurst just 6 miles away from Wadhurst, who built a 3.5-inch aperture transit telescope in an extension to his house on Blackheath close to Greenwich observatory.  From here he made some very accurate observations, compiling his own star catalogue down to 9th magnitude stars.

George Biddell Airy came next.  He was a distinguished mathematician and astronomer at Cambridge and moved to Greenwich in 1835 on being appointed Astronomer Royal.  When Groombridge suffered a stroke it was Airy who had had the work completed.  In fact Airy was very a very exacting supervisor of calculations and also improved measurements made by devising new instruments.

Last in Gilbert's list of important contributors to Positional Astronomy was Simon Newcomb, born in Newfoundland in 1835.  He moved to the United States and worked in the US Naval Observatory where he recalculated the positions of the planets and improved the astronomical constants.

Then we returned to the huge contributions made by George Airy.  He reduced the effects of atmospheric distortion on his measurements by using a wall mounted transit circle and by taking measurements using a bowl of mercury and comparing them with direct measurements and thus was able to reduce any errors.

Another instrument devised by Airy made use of the fact that there is no atmospheric distortion at or near the zenith.  The Reflex Zenith Tube worked by looking down onto the surface of mercury at the reflection a star which was within 2 or 3 degrees of the zenith.  Measurements were made when the star crossed various wires in the light path.  the instrument was turned and the reading taken again at 180 degrees.  Dividing this by 2 gave an exact position of the star.  These readings were required for the adjustment of the meridian instruments and measurement of the variation of latitude.

The Airy Transit Circle telescope was the last of the transit telescopes to be built and used at Greenwich, and this is the instrument that Gilbert was so familiar with.  The observer listens to a clock mechanism and as the object whose position was being measured crossed a series of wires in the optical path, he would tap on a "Morse key".  This would produce an impulse on a drum chronograph that would be measured later with an accuracy of 1/10 th of a second.

In 1915 accuracy was improved even more by the Impersonal Micrometer.  The observer tracked the star with a tracking wire.  As the tracking wire passed a fixed vertical wire, electrical contacts within the eyepiece produced in impulse that was recorded on the barrel chronograph, producing even more accurate results.

At the same time, the altitude angle was measured by and assistant, reading a high precision scale via one of a number of microscopes built through the side of the telescope mount.  Airy had also had an additional channel incorporated through the mount to allow light to pass through and illuminate the scale.

In 1939 a tickertape machine replaced the chronograph and resulted in readings accurate to 1/1000 th of a second.

Airy was retained as Astronomer Royal until he was 80!  He retired to a house just outside the walls of Greenwich Park and even then continued as a consultant to the observatory and served on the Board of Visitors.

Finally Gilbert spoke of more recent methods of measuring transits using a development of Airy's Zenith Tube utilising photographic techniques.  Using the PZT (Photographic Zenith Tube) extremely accurate results can be obtained.

The country of Copenhagen wanted a transit telescope at La Palma which was eventually paid for by the Carlesberg lager company, and was called until recently the "Carelsberg Automatic Meridian Telescope".  This is a fully computerised meridian telescope capable of very accurate measurements.

This telescope was the first fully automatic telescope in the world.  At present it is used for Astrometry.

Gilbert concluded his talk with a brief explanation of why the Global Positioning System (GPS) places the worlds reference meridian about a hundred metres to the east of Airy's Transit telescope and is marked by a litter bin.  In 1984 the World Geodetic System committee took the results of a number of surveys from around the world, averaging them and also taking into account the changes in gravity, plate movements and the oblateness of the earth's shape to determined the present meridian reference position known as WGS84.


The committee met in the Abergaveny Arms at Frant on Monday the 9th of July.

The Treasurer announced that the currant account contained 490.85 and our Reserve Account stood at 1,125.35.  This was considered to be healthy after the purchase of a slide projector and a new digital projector often required by more recent speakers.

Phil Berry spoke of a recent SAGAS (Southern Area Group of Astronomical Societies) meeting he had attended at Chichester and one of the subjects they spoke of was "The International Year of Astronomy".  This is to mark the 400th anniversary of Galileo and his telescope and will take place in 2009.

A number of events are being planned throughout the year such as Moon Week in July to celebrate the first telescope observations of the Moon in 1609.

The Committee discussed what our Society might do and it was felt that members may have some suggestions.  There is plenty of time, but we would like to have some ideas before the next Committee meeting this October.

The meeting concluded with a suggestion that if our trip to Belmont House in September is a success, the Society might arrange a trip to Greenwich Observatory and/or Herstmonceux.


August   As mentioned in previous Newsletters, there is no meeting of the Society in August, but once again, we have been kindly invited to an Astro Barbecue hosted by Michael Harte and his wife at Greenman Farm on Saturday 25th August 2007.  In the past this has been one of the Society's highlights of the year and promises to be again this year.

Michael and his wife, Claire, live in a remarkable old farmhouse with extensive grounds to the south.  The sky is fairly free from light pollution and apart from the occasional aircraft making its way to or from Gatwick, observing can be very rewarding.

Greenman Farm, Wadhurst, is on the south side of the B2099 immediately to the west of the railway over-bridge.  All Society members are invited and Michael suggests that members aim to arrive about 7.00 pm.  The entrance to the farm is through two huge gates and there is plenty of room inside for parking.

You will only need to bring your own food and drink, as everything else will be provided.

Members are invited to bring telescopes, binoculars and anything else of interest, but mainly themselves.

Michael adds a note of caution.  It is late August and can get a bit cold as the evening progresses, so it might be worth bringing some warm clothing.


Wednesday 19th September 2007   George Sallitt will be giving a talk about "Web cams", a subject that will interest many members keen to get involved with this cheaper but surprisingly satisfying method of imaging.

Saturday 22nd September 2007  A visit to see the largest private collection of clocks in the UK at Belmont House near Faversham in Kent.

Wednesday 17th October 2007  Keith Brackenborough will be giving a talk with the intriguing title "The Calendar - A 5,000 year struggle  to Align the Clock to the Heavens".

Wednesday 21st November 2007  John Vale-Taylor is presenting "The Tim Bance Interview".  Tim is a long-standing respected member of the Society and has a wealth of practical experience in the field of amateur astronomy.

Wednesday 12th December 2007  NOTE: THIS IS THE SECOND WEDNESDAY OF DECEMBER   Society Member, Paul Treadaway is giving a talk he calls "Why are we Still Here?" - Food for thought...




On Saturday the 22nd of September, The Society is visiting one of the finest collections of clocks in Britain.

We will need to find our own way there in our own transport. 

There will be detailed directions in the next Newsletter and also maps and instructions will be available at the Society's September meeting, which takes place just a few days before the visit.


A rather interesting opportunity has arisen due to automatic methods used to detect possible faint galaxies.  In New Mexico, Apache Point Observatory is carrying out a survey of a quarter of the sky, searching for faint galaxies.  They already have over a million images, but are asking help in identifying the different objects.

Chris Lintott, of Sky at Night programme fame, has helped to set up a website:   


on which you take a short training session, identifying different types of galaxies and then are invited to look at the real images, identify them to see if they are actual galaxies and then classifying them as edge on, right rotating, left rotating, colliding, etcetera.

I have spent a little while going through them and found it quite fascinating.  You will be looking at objects that no one else has ever seen until now and so far I have found one that certainly deserved further study.  You are invited to send an email containing the identification number to the observatory for further examination.  Each image is classified by 20 different assessors to help rationalise the results.

I certainly found it unusual - for a while.


Following the debate about thoughtless lighting in the Wadhurst area, Phil Berry notes that Wadhurst residential street lighting is now starting to be turned off after midnight until 0500. Apparently the majority of residents have seen sense and have voted to have it turned off.

There will be a formal review in April 2008. If the benefits are good, it would be as well to let the parish council know what we think ready for their review.  They are also looking at producing a lighting policy with a lighting contractor to review the 159 streetlights that are in the parish with regard to replacing the ones that are old and expensive to maintain.

Phil will be writing to them to encourage them to put in as low wattage and as "dark sky friendly" fittings as possible to meet the requirements. Perhaps other members in the area would like to do the same.

The Chairman of Highways & Lighting Committees is Anna Monaghan and The Wadhurst Clerk to the Council is Philippa Hewes and she can be emailed with a view to passing on comments via the email address:




Mercury is a morning object this month but will be low down and very difficult to spot.

Venus also becomes a morning object later in August and might be seen in the east before sunrise. It will present a thin crescent phase and will be magnitude -4.2.

Mars is in Taurus (the bull) and at magnitude 0.4 is growing in apparent diameter all the time. By the end of the month Mars rises before midnight (BST) and can be seen close to Aldebaran on the night of the 24th. 

Jupiter is in Ophiuchus (the serpent bearer) at magnitude -2.3. It is still a prominent evening object in the south west but will soon be setting before midnight.

Saturn is not visible this month due to its conjunction on the 21st.

Lunar Occultations

There are a few events this month that are listed below but there are also a series of occultations of the Pleiades during the early hours of August 7th. If you would like times for these please let me know. They start at 01.06 and end at 02.23. In total that morning there are 31 occultations although not all of them of the Pleiades. Times are all BST. DD = Disappearance on the Dark limb whilst RD = Reappearance on the Dark limb.
Date Time Star (SAO Catalogue) Constellation Magnitude Phase
Mon 20th Aug 2137 183485 Libra 7.5 DD
Thu 23rd Aug 2119 186444 Sagittarius 6.5 DD
Sun 26th Aug 2145 189986 Capricornus 4.9 DD
Mon 27th Aug 2248 164808 Aquarius 7.7 DD
Mon 27th Aug 2341 164809 Aquarius 7.6 DD
Tue 28th Aug 2344 146362 Aquarius 3.7 RD
Wed 29th Aug 2258 146862 Pisces 7.9 RD
Wed 29th Aug 2356 146885 Pisces 7.1 RD
Thu 30th Aug 2217 109282 Cetus 7.6 RD


As I said in the last Sky Notes, the Perseid meteor shower has its maximum this month on the night of 12th to 13th which fortunately coincides perfectly with new moon. Estimates of the ZHR (Zenithal Hourly Rate) vary but the average is around 80 meteors per hour. This method of estimating meteors can be quite confusing because it is a measure of the number that could be seen with a perfect sky and with the radiant at the zenith where there would no loss of seeing due to extinction.

Personally, I am quite happy to meteor watch simply for the enjoyment and relaxation that it offers but I know others prefer to do something more tangible.  If you want too you can record what you see by noting the date, time, magnitude estimate and the constellation where it was seen. Also if you have a star map you can draw on the meteors you have seen and then follow the trails back to show where the radiant is.

Around midnight the radiant lies in the north east but as the night progresses it moves to the east and by dawn (if you've lasted till then) it is high in the south/south east and the winter constellation of Orion will already be well above the horizon.

Lying on a sun lounger with the head end slightly raised and your feet pointing towards the radiant is the best position. Don't watch too close to the radiant - the best meteors are invariably seen further out over a wide field of view. Don't forget that even during "summer" when daytime temperatures can be high you will need a coat on if you watch for any period of time.

Perhaps we ought to arrange the WAS bar-b-q so that it coincides more closely with the Perseid maximum unless you think that's more likely to wash out both events!

ISS (International Space Station)

Below are just some of the best passes by the ISS this month. For more details log on to the web-site: - www.heavens-above.com    All times are in BST.

Date Magnitude Time Max Altitude Azimuth
3rd August -1.0 2152 21 SSE
4th August -2.0 2215 42 SSE
5th August -.2.5 2238 74 S
6th August -1.9 2152 41 SSE
6th August -2.4 2300 82 NNW
7th August 2.4 2148 73 SSE
8th August -2.3 2211 83 NNW
9th August -2.2 2233 76 NW
10th August -2.3 2121 83 N
10th August -2.5 2256 76 SW
11th August -2.3 2144 81 N
12th August -2.4 2206 76 SW
13th August -1.9 2229 45 SSW
14th August -2.4 2116 79 S
14th August -0.4 2251 21 WSW
15th August -1.8 2139 46 SSW
16th August -0.7 2201 24 SSW
17th August -1.8 2049 47 SSW
18th August -0.7 2111 24 SSW



Chairman   John Vale-Taylor  pjvalet@tiscali.co.uk

Phil Berry  01892 783544 phil.berry@tiscali.co.uk

Treasurer  Mike Wyles  01892 542863 mikewyles@globalnet.co.uk

Publicity & Website  Michael Harte  01892 783292 michael@greenman.demon.co.uk

Newsletter Editor  Geoff Rathbone  01959 524727 Geoff@rathbone007.fsnet.co.uk

Any material for inclusion in the September Newsletter should be with the Editor by August 28th  2007